The FF parameterizations considered are described in Table REF .
They include a Taylor expansion in the variable \(t/m_{\pi ^+}^2 \)  [1]}, where \(m_{\pi ^+}\)  is the charged pion mass,
a parameterization assuming vector and scalar pole masses \(M_V\)  and \(M_S\)  [2]}, [3]} and
a more physical dispersive parameterization [4]}.
The Taylor expansion is affected by large correlations between the measured parameters.
The pole parameterization has a physical interpretation for \(f_+ (t) \) 
related to the \(K^\ast (892)\)  scattering pole, but not for \(f_0 (t) \)  with no corresponding pole.
The dispersive parameterization makes use of general chiral symmetry and analyticity constraints, and external inputs from \(K\) –\(\pi \)  scattering data, via
the functions \(H(t)\)  and \(G(t)\) :
\(\begin{aligned}G(t) &\: = \: x \cdot G_{p1} + (1-x) \cdot G_{p2} + x \cdot (1-x) \cdot G_{p3}, \\H(t) &\: = \: x \cdot H_{p1} + x^2 \cdot H_{p2},\end{aligned}\) 
